This invention relates to mixtures of 1,1,1,2-tetrafluoroethane (HFC-134a) and dimethyl ether (DME). Such mixtures are useful as refrigerants, heat transfer media, gaseous dielectrics, foam expansion agents, aerosol propellants and power cycle working fluids. These mixtures are potentially environmentally safe substitutes for dichlorodifluoromethane (CFC-12), which is a large volume commercial refrigerant.
Closed-cell polyurethane foams are widely used for insulation purposes in building construction and in the manufacture of energy efficient electrical appliances. In the construction industry, polyurethane (polyisocyanurate) board stock is used in roofing and siding for its insulation and load-carrying capabilities. Poured and sprayed polyurethane foams are widely used for insulating large structures such as storage tanks, etc. Pour-in-place polyurethane foams are used, for example, in appliances such as refrigerators and freezers plus they are used in making refrigerated trucks and railcars.
All of these various types of polyurethane foams require expansion agents (blowing agents) for their manufacture. Insulating foams depend on the use of halocarbon blowing agents, not only to foam the polymer, but primarily for their low vapor thermal conductivity, a very important characteristics for insulation value. Historically, polyurethane foams are made with CFC-11 (CFCl.sub.3) as the primary blowing agent.
A second important type of insulating foam is phenolic foam. These foams, which have very attractive flammability characteristics, are generally made with CFC-11 and CFC-113 (1,1,2-trichloro-1,2,2-trifluoroethane) blowing agents.
A third type of insulating foam is thermoplastic foam, primarily polystyrene foam. Polyolefin foams (polyethylene and polypropylene) are widely used in packaging. These thermoplastic foams are generally made with CFC-12.
Many refrigeration applications, e.g., refrigerators and auto air conditioners, presently use CFC-12 as the refrigerant. HFC-134a is a fluorocarbon compound identified as a potential replacement for CFC-12, having properties near those of CFC-12, and being nonflammable with a zero ozone depletion potential. However, HFC-134a has a lower refrigeration coefficient of performance than that of CFC-12, which can result in increased energy consumption for refrigeration applications if HFC-134a is substituted for CFC-12.
Many products designed for household, personal or industrial use are available as aerosol products. Typical examples of such products and ones in which the propellant system of the present invention can be used included personal products such as hair sprays, deodorants and colognes; household products such as waxes, polishes, pan sprays, room fresheners and household insecticides; industrial products such as cleaners, lubricants and mold release agents; and automotive products such as cleaners and polishers. All such products utilize the pressure of a propellant gas or a mixture of propellant gases (i.e., a propellant gas system) to expel the active ingredients from the container. For this purpose, most aerosols employ liquified gases which vaporize and provide the pressure to propel the active ingredients when the valve on the aerosol container is pressed open.
An important physical property associated with the dispensing of aerosol products is the vapor pressure of the propellant. Vapor pressure from the viewpoint of this invention is the pressure exerted when a liquified propellant gas is in equilibrium with its vapor in a closed container, such as an aerosol can. Vapor pressure can be measured by connecting a pressure gauge to the valve on an aerosol can or gas cylinder containing the vapor/liquid mixture. A standard of measurement of vapor pressure in the U.S. aerosol industry is pounds per square inch gauge (psig) with the gas/liquid mixture at constant temperature, most commonly at 70.degree. F. (21.degree. C.). The vapor pressure of liquified gases most widely employed as aerosol propellants will vary over the range of about 20 to 90 psig (138 to 621 kPa) at 70.degree. F. (21.degree. C.). The propellant systems of the present invention have vapor pressures in this range.
In the early 1970s, concern began to be expressed that the stratospheric ozone layer (which provides protection against penetration of the earth's atmosphere by ultraviolet radiation) was being depleted by chlorine atoms introduced to the atmosphere from the release of fully halogenated chlorofluorocarbons. These chlorofluorocarbons are used as propellants in aerosols, as blowing agents for foams, as refrigerants and as cleaning/drying solvent systems. Because of the great chemical stability of fully halogenated chlorofluorocarbons, according to the ozone depletion theory, these compounds do not decompose the earth's atmosphere but reach the stratosphere where they slowly degrade liberating chlorine atoms which in turn react with the ozone.
Concern reached such a level that in 1978 the U.S. Environmental Protection Agency (EPA) placed a ban on nonessential uses of fully halogenated chlorofluorocarbons as aerosol propellants. This ban resulted in a dramatic shift in the U.S. away from chlorofluorocarbon propellants (except for exempted uses) to primarily hydrocarbon propellants. However, since the rest of the world did not join the U.S. in this aerosol ban, the net result has been to shift the uses of chlorofluorocarbons in aerosols out of the U.S., but not to permanently reduce the world-wide total chlorofluorocarbon production, as sought. In fact, in the last few years the total amount of chlorofluorocarbons manufactured worldwide has exceeded the level produced in 1978 (before the U.S. ban).
During the period of 1978-1987, much research was conducted to study the ozone depletion theory. Because of the complexity of atmospheric chemistry, many questions relating to this theory remain unanswered. However, assuming the theory to be valid, the health risks which would result from depletion of the ozone layer are significant. This, coupled with the fact that world-wide production of chlorofluorocarbons has increased, has resulted in international efforts to reduce chlorofluorocarbon use. Particularly, in September, 1987, the United Nations through its Environment Programme (UNEP) issued a tentative proposal calling for a 50 percent reduction in world-wide production of fully halogenated chlorofluorocarbons by the year 1998. This proposal was ratified Jan. 1, 1989 and became effective on Jul. 1, 1989.
Because of this proposed reduction in availability of fully halogenated chlorofluorocarbons such as CFC-11, CFC 12 and CFC-113, alternatively more environmentally acceptable products are urgently needed.
As early as the 1970s with the initial emergence of the ozone depletion theory, it was known that the introduction of hydrogen into previously fully halogenated chlorofluorocarbons markedly reduced the chemical stability of these compounds. Hence, these now destabilized compounds would be expected to degrade in the atmosphere and not reach the stratosphere and the ozone layer. The following Table I lists the ozone depletion potential for a variety of fully and partially halogenated halocarbons. Halocarbon Global Warming Potential data (potential for reflecting infrared radiation (heat) back to earth and thereby raising the earth's surface temperature) are also shown.
TABLE I ______________________________________ Ozone Depletion and Halocarbon Global Warming Potentials Halocarbon Ozone Depletion Global Warming Blowing Agent Potential Potential ______________________________________ CFC-11 (CFCl.sub.3) 1.0 1.0 CFC-12 (CF.sub.2 Cl.sub.2) 1.0 2.8 HCFC-22 (CHF.sub.2 Cl) 0.05 0.3 HCFC-123 (CF.sub.3 CHCl.sub.2) 0.02 0.02 HCFC-124 (CF.sub.3 CHFCl) 0.02 0.09 HFC-134a (CF.sub.3 CH.sub.2 F) 0 0.3 HCFC-141b (CFC1.sub.2 CH.sub.3) 0.1 0.09 HCFC-142b (CF.sub.2 ClCH.sub.3) 0.06 0.3 HFC-152a (CHF.sub.2 CH.sub.3) 0 0.03 CFC-113 (CF.sub.2 Cl--CFCl.sub.2) 0.8-0.9 1.4 ______________________________________
Halocarbons such as HFC-134a have zero ozone depletion potential. Dimethyl ether, having no halogen content, is also a zero ozone depleter.
Although 1,1,1,2-tetrafluoroethane has utility as a refrigerant, aerosol propellant or foam blowing agent, an azeotrope offers the possibility of producing more economical nonfractionating systems with improved properties such as polymer and refrigerant oil solubility.
Unfortunately, as recognized in the art, it is not possible to predict the formation of azeotropes.
This fact obviously complicates the search for new azeotropes which have application in the field. Nevertheless, there is a constant effort in the art to discover new azeotropic compositions, which have desirable characteristics.